All non-reusable radioactive by-products or residues of processes, or more generally of operations, in which radioactive substances have been generated or used, are identified as “nuclear waste”. Owing to its hazardousness for humans and the environment, nuclear waste of any type and origin must be treated and stored according to quite particular methodologies, which ensure that the radiation and nuclear elements or isotopes are confined even for very long periods of time.
There are numerous types of processes in which nuclear elements or radiation are used, which produce waste at various levels of concentration and hazardousness. A proposed classification, in use in Italy, divides these wastes into:                category 1, which comprises all waste with a low level of radioactivity; it is the largest category, comprising, by weight, approx. 90% of the waste produced, but only 1% of the radioactivity (examples are medical material used in nuclear medicine, disposable clothing supplied during a visit to a nuclear plant etc.);        category 2, which comprises all waste with a medium level of radioactivity; this requires shielding, but only constitutes 7% of the wastes, with a total radioactivity of 4% (examples are the sheaths of the fuel elements of a reactor);        category 3, which comprises all wastes with a high level of radioactivity, constituting only 3% of the wastes but representing 95% of the radioactivity; they are the most dangerous owing to the high radiation dose that accidental exposure would involve and owing to the decay of the order of millions of years for some of the radioactive isotopes that they contain.        
The different types of waste require different disposal procedures. Numerous techniques for this purpose have been investigated and described in the last 60 years. The results are in the public domain and in general are easily accessible; for the specific methods relating to the long-term storage of types of waste containing long-lived and/or highly mobile isotopes, the conclusions are, however, still uncertain. The resources invested in these studies are, presumably, enormous; those to be invested for the conditioning and long-term storage of existing nuclear waste (including reclamation of the associated sites), are known in part: for the USA alone they have been evaluated at hundreds of billions of dollars.
The disposal of these wastes generally requires a phase of conditioning, which consists in transforming the waste to a form suitable for storage; and storage of the conditioned waste at suitable sites, either natural or produced industrially.
A particular type of nuclear waste, strategically very important, is that generated in the operations for reclamation of nuclear reactors that are no longer active and of nuclear sites that have become obsolete. In this case the nuclear wastes are typically generated in the operations of recovery and decontamination of large metal structures, which, exposed to contact and/or to the radiation of radioactive isotopes, have in their turn become radioactive (limited to the exposed surface), by chemical contamination or by nuclear mutation (under the action of radiation). The complex of operations associated with these reclamation operations is called “decommissioning” in this field, and this term will be used hereinafter. The dominant technique in the operations of decontamination of metallic surfaces is called “pickling”.
Many of the wastes from decommissioning thus generated belong to the aforementioned category 3, and typically contain isotopes with long average life and of high mobility, which always require the specific conditioning for highly hazardous waste. While processes that are industrially approved and economically viable have been identified for the complete management of nuclear waste belonging to categories 1 and 2, for those of category 3 the results obtained are important but still partial, especially for the uneconomic aspect of the conditioning required, and to date there is no operational depository for long-term storage.
With regard to the conditioning of the decommissioning waste, and typically that generated by pickling, the experts have come to the conclusion that it is necessary to use vitreous matrices with high stability, both chemical and thermo-mechanical, for all long-lived and/or highly mobile radioactive isotopes; see for example the article “Glass packages guaranteed for millions of years”, by É. Y. Vernaz, Clefs CEA, No. 46 (2002), p. 81-84. Numerous examples of vitrification of category 3 waste, including at industrial level, have been proposed, but they were beset by problems of process reliability and typically high costs.
Recently, among the most promising vitreous materials for the purpose of retaining radioactive isotopes, especially if in the presence of sulphates, chromates, phosphates and halides, phosphate vitreous systems containing iron have been accredited. Systems of this type are described in U.S. Pat. No. 5,750,824 and U.S. Pat. No. 5,840,638 and in patent application GB 2,371,542 A.
Among these, U.S. Pat. No. 5,750,824 is particularly interesting, and teaches the production of phosphate glasses containing from 30 to 70 wt. % of phosphorus oxide (as P2O5) and from 22 to 50% of iron oxide, the rest consisting of oxides of other metals, including those derived from nuclear waste; moreover, this document teaches that the best results are obtained with glasses in which iron is present to at least 50%, preferably at least 80% and more preferably at least 90%, in oxidation state 3, i.e. as Fe3+ ion. According to this document, phosphate glasses with a high Fe3+/Fe2+ ratio are characterized by the best properties of chemical resistance (for example, to leaching, i.e. washing away with water), of density and of thermomechanical resistance.
The methods taught in these documents envisage the preparation of a mixture of powders of oxides or salts of phosphorus and of iron in the desired weight ratios; melting of this mixture; addition, before or during said melting, of the waste to be disposed of; and solidification of the melt in suitable moulds.
A problem that is still open with these methods is management of the huge volumes of liquid of the solutions in which the decommissioning waste is initially dissolved. In fact, in some cases the solutions are added directly to the melt of oxides or salts of phosphorus and iron, generating however enormous volumes of vapours that must then be condensed, decontaminated and disposed of; in other cases, the solutions are first dried, and the waste is added in the form of powder to the melt, but again in this case obtaining the powders of waste involves evaporation of large amounts of liquid.